The invention relates to an internal axis, single-rotation machine, with an external rotor and an internal rotor sealingly enclosed by a common housing, which rotate in a speed ratio of 2:3 about axes having a constant eccentricity, relative to each other (i.e., a constant distance between the axes of external and internal rotors) so that the external rotor has three engagement parts with a radial width corresponding to twice the eccentricity and the internal rotor two engagement parts, the internal rotor and the external rotor being in permanent reciprocal engagement during rotation.
A constant mutual engagement is necessary in order to obtain a satisfactory efficiency of such a single-rotation machine. However, this means that both rotors approach at the engagement points only up to a very narrow spacing forming a sealing gap. A direct engagement contact would lead to rotor damage in the case of the high rotational speeds sought for gas feed purposes. The minimum possible width of the sealing gap at the engagement points is determined by the precision of manufacture of the rotors and the accuracy of their installation.
The kinematically exact shape of the rotor parts coming into mutual engagement necessary for a minimum sealing gap width can only be manufactured with considerable effort and expenditure in the case of known single-rotation machines of this type. A machine of the aforementioned type, which is characterized by a particularly good volumetric efficiency is known from EP-A-167 846 (Wankel).
In order to reduce the manufacturing costs U.S. Pat. No. 1,753,476 (Richer) proposes making the inner face of the engagement parts of the external rotor arcuate and the internal rotor ring-shaped, i.e. bounded by two circular arcs and two straight lines. As a result of this shaping it is not possible to obtain a precise kinematic shape of the rotors, so that due to intermittent mutual engagement of the rotors or varying and excessive engagement gaps, it would only be appropriate for use in connection with machines for feeding liquid media and having correspondingly low speeds. Such a shape would correspond to a spacing of the center of the circle of the arc of the external rotor from its center corresponding to eight times the eccentricity and with a radius of curvature corresponding to six times the eccentricity.